1. Field of the Invention
The present invention relates generally to methods of coating a non-wettable material onto a substrate, methods for making ceramic-metal structures, such as tapes, methods for bonding two ceramic bodies together, and, structures formed by the methods.
2. Description of the Related Art
With today's increasing needs for improved materials, there are often instances when it would be desirable to have a material with properties of a metal, such as toughness, deformability, electrical conductivity, heat conductivity, and welding capability, and the strength and rigidity of a ceramic. One way of achieving such a hybrid material would be to layer a metal and a ceramic by coating a metal onto a ceramic substrate. The resulting layered material could be used to make conventional devices, but since the material would combine the advantages of a metal with a ceramic, the device could be made smaller, thus, lighter, than devices made of previous materials and yet remain efficacious for the intended use.
However, it is often found that a chosen metal does not coat well onto a chosen ceramic because the wetting properties between the chosen metal and ceramic are not conducive for wetting.
It would also be desirable to coat a nonwettable metal onto a ceramic substrate and resulting in a minimal amount of metal oxide in the layered product, as metal oxides are often detrimental to conductivity and strength.
There are also countless other instances when it is difficult to coat a material, whether the material be solid or liquid at room temperature, onto a substrate of choice because the wetting characteristics between the material to be coated and the substrate material are not conducive for wetting.
In the more specific field of making ceramiemetal tapes, the most common method used is a tape casting technique. This technique often requires a high content of binders in the casted tape, which is undesirable because the binders are frequently difficult to remove. The tape casting technique also often requires the use of organic solvents in order to cast the tapes. However, the use of organic solvents is not environmentally desirable.
Another problem with current tape casting techniques is that developing the formulations suitable for both the ceramic and the metal powders is not an easy task. In addition, it is difficult to control the thickness of tapes formed by current techniques. A further problem with current tape casting techniques is that tapes that are formed are porous and shrink in all dimensions upon densification at elevated temperatures and typically require undesirably high temperatures for processing.
In the ceramics industry, there is also a need for improved ways of bonding two ceramic bodies together. Often, two ceramic bodies are bonded together with another ceramic. The thus-formed ceramic joint has good strength, but once the joint fails, it fails in a catastrophic manner. When a metal joint is used to bond two ceramic bodies together, the metal joint typically has a lower strength than a ceramic joint, but is capable of deforming with slow crack propagation and, therefore, has less catastrophic failure than a ceramic joint. Therefore, it is desirable to have a way of bonding two ceramic bodies together which has the advantage of the high strength of a ceramic joint as well as the characteristically less catastrophic failure of a metal joint.
It is, therefore, one object of the present invention to provide a structure of a non-wettable material on a solid substrate and a method for making same.
It is yet another object of the present invention to provide a method for making ceramic-metal structures, such as tapes, which method requires little or no binders or organic solvents, is applicable to many different ceramic-metal formulations, allows easy control of the thickness of the tape, forms fairly dense tapes so that little or no shrinkage occurs upon densification at elevated temperatures, and does not require undesirably high processing temperatures.
It is still another object of the present invention to provide a structure of two ceramic bodies bonded together having high bond strength and slow crack propagation and a method of bonding the two ceramic bodies together.